CN111785748A - Method and structure for reducing dark current of backside illuminated image sensor - Google Patents

Abstract

The present invention provides a method and structure for reducing the dark current of a backside illuminated image sensor. The method includes the following steps: 1) providing an SOI substrate, the SOI substrate comprising a silicon substrate, a buried oxide layer and a top layer of silicon; 2) forming a pinning layer by doping in the top layer silicon; 3) forming an epitaxial layer over the top layer silicon and forming a photodiode in the epitaxial layer; 4) using the buried oxide layer as a thinning A stop layer is used for backside thinning of the silicon substrate. In the present invention, a pinned layer is formed on the top silicon of the SOI substrate, and the buried oxide layer is used as a thinning stop layer in the backside thinning process. By introducing the pinning layer as the charge trapping layer of the photodiode, the probability of the electric charge flowing into the photodiode to form dark current is reduced, and the pinning layer can be protected by the buried oxide layer in the thinning process. In addition, by cooperating with the backside growth of a high-K dielectric layer, the interface dark current is further reduced.

Description

Method and structure for reducing dark current of backside illuminated image sensor

technical field

The invention relates to the field of semiconductor integrated circuit manufacturing, in particular to a method and structure for reducing the dark current of a backside illuminated image sensor.

Background technique

In the manufacturing process of the backside illuminated image sensor, the silicon substrate needs to be thinned by a backside thinning process. Defects, dangling bonds and damage on the exposed silicon substrate surface after thinning will lead to dark current on the silicon surface. The surface dark current of the back-illuminated image sensor will cause the noise of the back-illuminated image sensor to increase sharply compared with the front-illuminated image sensor, the image quality will be greatly reduced, and it will even be difficult to image effectively.

At present, in order to reduce the surface dark current of the back-illuminated image sensor, processes such as setting a conductive structure on the silicon surface and applying a negative bias voltage have been used to form hole enrichment on the silicon surface and improve the surface dark current.

However, the existing improvement process still has the disadvantages of poor improvement effect, complex process integration and high cost, which cannot meet the process requirements of the advanced back-illuminated image sensor process.

Therefore, it is necessary to propose a new method and structure for reducing the dark current of the back-illuminated image sensor to solve the above problems.

SUMMARY OF THE INVENTION

In view of the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide a method and structure for reducing the dark current of a back-illuminated image sensor, so as to solve the problem that the surface dark current of the back-illuminated image sensor affects the performance of the device in the prior art. question.

In order to achieve the above object and other related objects, the present invention provides a method for reducing the dark current of a backside illuminated image sensor, which is characterized in that it includes the following steps:

1) providing an SOI substrate, the SOI substrate includes a silicon substrate, a buried oxide layer and a top layer silicon;

2) forming a pinning layer formed by doping in the top layer silicon;

3) forming an epitaxial layer over the top layer silicon, and forming a photodiode in the epitaxial layer;

4) Using the buried oxide layer as a thinning stop layer, perform backside thinning on the silicon substrate.

As an optional solution of the present invention, the doping type of the pinning layer is opposite to that of the photodiode.

As an optional solution of the present invention, the method for forming the pinning layer includes ion implantation into the top layer silicon.

As an optional solution of the present invention, the doping dose of the ion implantation is 5×10 ¹² to 5×10 ¹⁴ cm ⁻² .

As an optional solution of the present invention, the doping concentration of the epitaxial layer is 1×10 ¹⁴ to 5×10 ¹⁵ cm ⁻³ , and the thickness of the epitaxial layer is 2.5 to 10 μm.

As an optional solution of the present invention, the thickness of the buried oxide layer is greater than 1 μm.

As an optional solution of the present invention, after the photodiode is formed in step 3), a step of forming a metal wiring layer over the epitaxial layer is also included.

As an optional solution of the present invention, a top pinning layer is further formed between the epitaxial layer and the metal wiring layer.

As an optional solution of the present invention, before the backside thinning of the silicon substrate in step 4), the step of bonding the side of the SOI substrate away from the silicon substrate to a carrier is also included .

As an optional solution of the present invention, in step 4), the buried oxide layer is used as a thinning stop layer, and after the backside thinning is performed on the silicon substrate, the method further includes removing the thinning on the silicon substrate. The remaining buried oxide layer is then formed, and a high-K dielectric layer, a color filter layer and a microlens structure are sequentially formed on the exposed pinned layer.

As an optional solution of the present invention, there are multiple photodiodes, and after removing the remaining buried oxide layer after thinning the silicon substrate, the method further includes forming a plurality of partitions in the epitaxial layer. The steps of the backside deep trench isolation structure of the photodiode.

The present invention also provides a structure for reducing the dark current of the back-illuminated image sensor, which is characterized by comprising:

epitaxial layer;

a photodiode formed in the epitaxial layer;

a top layer of silicon over the epitaxial layer;

A pinning layer, which is located in the top layer silicon, is formed by doping the top layer silicon.

As an optional solution of the present invention, the doping type of the pinning layer is opposite to that of the photodiode.

As an optional solution of the present invention, the doping concentration of the epitaxial layer is 1×10 ¹⁴ to 5×10 ¹⁵ cm ⁻³ , and the thickness of the epitaxial layer is 2.5 to 10 μm.

As an optional solution of the present invention, a high-K dielectric layer, a color filter layer and a microlens structure are formed on the pinning layer in sequence.

As an optional solution of the present invention, a metal wiring layer is further formed under the epitaxial layer.

As an optional solution of the present invention, a top pinning layer is further formed between the epitaxial layer and the metal wiring layer.

As an optional solution of the present invention, there are multiple photodiodes, and a backside deep trench isolation structure separating the multiple photodiodes is formed in the epitaxial layer.

As an optional solution of the present invention, the top layer silicon is a part of an SOI substrate, and the SOI substrate further includes a silicon substrate and a buried oxide layer; the buried oxide layer is located above the top layer silicon, and the The silicon substrate is located above the buried oxide layer; when the backside thinning of the silicon substrate is performed, the buried oxide layer is used as a thinning stop layer.

As described above, the present invention provides a method and structure for reducing the dark current of a backside illuminated image sensor, which has the following beneficial effects:

In the present invention, a pinned layer is formed on the top silicon of the SOI substrate, and the buried oxide layer is used as a thinning stop layer in the backside thinning process. By introducing the pinning layer as the charge trapping layer of the photodiode, the probability of the electric charge flowing into the photodiode to form dark current is reduced, and the pinning layer can be protected by the buried oxide layer in the thinning process. In addition, by cooperating with the backside growth of a high-K dielectric layer, the interface dark current is further reduced.

Description of drawings

FIG. 1 is a flowchart of a method for reducing the dark current of a backside-illuminated image sensor provided in Embodiment 1 of the present invention.

FIG. 2 is a schematic cross-sectional view of the SOI substrate provided in Embodiment 1 of the present invention.

FIG. 3 is a schematic cross-sectional view after forming the pinning layer provided in the first embodiment of the present invention.

FIG. 4 is a schematic cross-sectional view after forming an epitaxial layer according to Embodiment 1 of the present invention.

FIG. 5 is a schematic cross-sectional view after forming the photodiode provided in the first embodiment of the present invention.

FIG. 6 is a schematic cross-sectional view after forming a metal wiring layer according to Embodiment 1 of the present invention.

FIG. 7 is a schematic cross-sectional view of the first embodiment of the present invention after being bonded to the carrier sheet.

FIG. 8 is a schematic cross-sectional view after grinding and thinning and removing the buried oxide layer provided in Embodiment 1 of the present invention.

FIG. 9 is a schematic cross-sectional view of the backside deep trench isolation structure provided in Embodiment 1 of the present invention.

FIG. 10 is a schematic cross-sectional view of the color filter layer and the microlens structure provided in the first embodiment of the present invention.

FIG. 11 is a schematic cross-sectional view of the backside illuminated image sensor provided with the top pinned layer provided in Embodiment 2 of the present invention.

FIG. 12 is a schematic cross-sectional view of a back-illuminated image sensor provided in Embodiment 3 of the present invention without forming a high-K dielectric layer.

Component label description

101 SOI substrate

101a silicon substrate

101b Buried Oxygen Layer

101c top layer silicon

102 pinned layer

103 Epitaxial layer

104 Photodiode

105 Metal wiring layer

105a Metal Interconnect Structure

105b Intermetal dielectric layer

106 slides

107 High-K dielectric layer

108 color filter layers

109 Micro lens structure

110 Backside Deep Trench Isolation Structure

202 pinned layer

203 Epitaxial layer

204 Photodiode

205 Metal wiring layer

206 slides

207 High-K dielectric layer

208 color filter layers

209 Micro lens structure

210 Backside Deep Trench Isolation Structure

211 Top pinned layer

302 pinned layer

303 Epitaxial layer

304 Photodiode

305 metal wiring layer

306 slides

307 High-K Dielectric Materials

308 color filter layers

309 Micro lens structure

310 backside deep trench isolation structure

Detailed ways

The embodiments of the present invention are described below through specific specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

See Figures 1 to 12. It should be noted that the diagrams provided in this embodiment are only to illustrate the basic concept of the present invention in a schematic way, although the diagrams only show the components related to the present invention rather than the number, shape and the number of components in actual implementation. For dimension drawing, the shape, quantity and proportion of each component can be arbitrarily changed during actual implementation, and the component layout shape may also be more complicated.

Example 1

Referring to FIG. 1 to FIG. 10 , this embodiment provides a method for reducing the dark current of a backside-illuminated image sensor, which is characterized by including the following steps:

1) providing an SOI substrate 101, the SOI substrate 101 sequentially includes a silicon substrate 101a, a buried oxide layer 101b and a top layer silicon 101c from bottom to top;

2) forming a pinning layer 102 formed by doping in the top layer silicon 101c;

3) forming an epitaxial layer 103 over the top layer silicon 101c, and forming a photodiode 104 in the epitaxial layer 103;

4) Using the buried oxide layer 101b as a thinning stop layer, the backside thinning of the silicon substrate 101a is performed.

In step 1), referring to step S1 of FIG. 1 and FIG. 2, an SOI substrate 101 is provided, the SOI substrate 101 includes a silicon substrate 101a, a buried oxide layer 101b and a top layer silicon 101c sequentially from bottom to top.

As an example, the SOI substrate 101 is a silicon-on-insulator (Silicon-On-Insulator), which can be prepared by a smart lift-off process or an oxygen injection isolation process. Optionally, the thickness of the buried oxide layer 101b is greater than 1 μm, so as to ensure that when the backside grinding is thinned, the buried oxide layer 101b has a sufficient thickness as a thinning stop layer and can protect the pinning layer covered by the buried oxide layer.

In step 2), referring to step S2 in FIG. 1 and FIG. 3, a pinning layer 102 formed by doping is formed in the top layer silicon 101c.

As an example, a method of forming the pinned layer 102 includes ion implantation of the top layer silicon 101c. The doping dose of the ion implantation is 5×10 ¹² to 5×10 ¹⁴ cm ⁻² . In this embodiment, the ion implantation range covers the entire thickness of the top layer silicon 101c, so that it is completely converted into the pinning layer 102 after ion implantation, while in other embodiments of the present invention, when the When the thickness of the top layer silicon is relatively thick, according to device design and process requirements, only a part of the depth of the top layer silicon can be converted to the pinning layer.

As an example, the doping type of the pinning layer 102 is opposite to the doping type of the subsequently formed photodiode. For example, when the doping type of the photodiode is N-type, the pinning layer 102 with P-type doping is formed by ion implantation. It acts as a pinning layer on the surface of the photodiode. The pinned structure forms a hole-rich region on the surface of the silicon, which acts as a trap layer on the surface of the photodiode to realize the rapid recombination of electrons generated at the interface. Thus, electrons formed at the interface are prevented from entering the photodiode to form a dark current signal. This will effectively reduce the dark current on the silicon surface, solve the problem of noise, and then improve the imaging quality of the back-illuminated image sensor.

In step 3), referring to step S3 of FIG. 1 and FIGS. 4 to 6 , an epitaxial layer 103 is formed over the top layer silicon 101 c , and a photodiode 104 is formed in the epitaxial layer 103 .

As shown in FIG. 4, an epitaxial layer 103 is formed over the top layer silicon 101c. The epitaxial layer 103 may be a high-resistance silicon epitaxial layer for preparing photodiodes. Optionally, the doping concentration of the epitaxial layer 103 is 1×10 ¹⁴ to 5×10 ¹⁵ cm ⁻³ , and the thickness of the epitaxial layer 103 is 2.5 to 10 μm.

As shown in FIG. 5 , a photodiode 104 (PD, Photo-Diode) is formed in the epitaxial layer 103 obtained by epitaxial growth in FIG. 4 . The structure and preparation process of the photodiode 104 may refer to the prior art, and FIG. 5 only schematically shows its position in the epitaxial layer 103 . Optionally, a photodiode isolation structure (PDI, Photo-Diode Isolation) not shown in the figure is further formed on the side of the photodiode 104 to isolate a single photodiode from its surrounding epitaxial layers.

As an example, as shown in FIG. 6 , after the photodiode 104 is formed, a step of forming a metal wiring layer 105 over the epitaxial layer 103 is further included. The metal wiring layer 105 includes a metal interconnection structure 105a for providing electrical connection and an intermetal dielectric layer 105b for insulation. The metal wiring layer 105 is used to electrically connect sensor devices such as photodiodes and draw out electrical signals.

In step 4), referring to step S4 in FIG. 1 and FIGS. 7 to 10 , the backside thinning of the silicon substrate 101 a is performed using the buried oxide layer 101 b as a thinning stop layer.

As shown in FIG. 7 , the device structure obtained in FIG. 6 is inverted; before the backside thinning of the silicon substrate 101a, it also includes bonding the side of the SOI substrate 101 away from the silicon substrate 101a steps on slide 106 . The carrier 106 serves as a supporting substrate, which can provide support for the thinned wafer in subsequent processes such as thinning to prevent debris caused by stress. Specifically, in this embodiment, the metal wiring layer 105 is bonded to the carrier sheet 106 with the side facing down after being inverted. In addition, in addition to the carrier sheet only having a support function, the carrier sheet may also be a functional sheet with other device structures formed, and a complete image sensor device is formed by bonding with the SOI substrate.

As shown in FIGS. 7 to 8 , the backside thinning of the silicon substrate 101a is performed by using the buried oxide layer 101b as a thinning stop layer. As shown in FIG. 8 , after grinding and thinning, the remaining buried oxide layer may be removed by wet etching and other methods to expose the intact and undamaged underlying pinning layer 102 for subsequent processing.

As shown in FIG. 9 , there are multiple photodiodes 104 , and after removing the buried oxide layer 101 b remaining after thinning the silicon substrate 101 a , it also includes forming a plurality of partitions in the epitaxial layer 103 . The steps of the back-side deep trench isolation structure 110 (BDTI, Back-side Deep Trench Isolation) of the photodiode 104 . The deep trenches of the backside deep trench isolation structure 110 may be formed by an anisotropic dry etching process, and an insulating dielectric material is further filled in the deep trenches to form the backside deep trench isolation structure 110. In this embodiment, the insulating dielectric material filled in the deep trench is a high-K dielectric material. Specifically, as shown in FIG. 9 , after forming the backside deep trench isolation structure 110 , the process further includes forming a high-K dielectric layer 107 in the backside deep trench isolation structure 110 and on the pinning layer 102 . step. Optionally, the high-K dielectric layer 107 is composed of a high-K dielectric material such as AlO or HfO. In this embodiment, by introducing the high-K dielectric layer 107 to cooperate with the pinning layer 102 , the combined action of the high-K dielectric layer 107 can enhance the hole-rich region on the silicon surface, so as to further reduce the dark current.

As shown in FIG. 10 , after the high-K dielectric layer 107 is formed, a step of sequentially forming a color filter layer 108 and a microlens structure 109 on the high-K dielectric layer 107 is included. Optionally, the color filter layer 108 may be divided into red (R), green (G) and blue (B) filter layers. The number, grouping, and arrangement of different color filter layers can be adjusted to the specific design of the sensor. For sensors with other wavelength bands such as non-visible light, other types of filter layers can also be set accordingly. It should also be pointed out that, in the embodiment of the present application, the back side of the back-illuminated image sensor refers to the side where the color filter layer 108 and the microlens structure 109 are located, and the front side refers to the side opposite to it. The other side where the metal wiring layer 105 is located.

As shown in FIG. 10 , this embodiment provides a structure for reducing the dark current of a backside illuminated image sensor, which is characterized by comprising:

epitaxial layer 103;

a photodiode 104 formed in the epitaxial layer 103;

top layer silicon 101c, which is located above the epitaxial layer 103;

The pinning layer 102, which is located in the top layer silicon 101c, is formed by doping the top layer silicon 101c.

As an example, the doping type of the pinning layer 102 is opposite to the doping type of the photodiode 104 . The doping concentration of the epitaxial layer 103 is 1×10 ¹⁴ to 5×10 ¹⁵ cm ⁻³ , and the thickness of the epitaxial layer 103 is 2.5 to 10 μm.

As an example, as shown in FIG. 10 , a high-K dielectric layer 107 , a color filter layer 108 and a microlens structure 109 are formed on the pinning layer 102 in sequence. A metal wiring layer 105 is also formed under the epitaxial layer 103 . There are a plurality of photodiodes 104 , and a backside deep trench isolation structure 110 is formed in the epitaxial layer 103 to separate the plurality of photodiodes 104 .

As an example, as shown in FIG. 7 , this embodiment also provides a structure for reducing dark current of a backside illuminated image sensor, which is characterized in that it includes an epitaxial layer 103 , a photodiode 104 and a pinning layer 102 . The top layer silicon 101c is a part of the SOI substrate 101, and the SOI substrate 101 further includes a silicon substrate 101a and a buried oxide layer 101b; the buried oxide layer 101b is located above the top layer silicon 101c, and the silicon The substrate 101a is located above the buried oxide layer 101b; when the backside thinning of the silicon substrate 101a is performed, the buried oxide layer 101b is used as a thinning stop layer.

In this embodiment, the pinning layer 102 is formed by ion implantation, and its doping concentration is adjustable, and it is not necessary to add a high-temperature annealing process for the doping in the backside process. As a charge trapping layer on the surface of the photodiode, it reduces the probability that charges flow into the photodiode to form dark current; and the high-K dielectric layer 107 is grown on the back of the device to minimize the interface dark current. The process method of this implementation solves the problem of noise, thereby improving the imaging quality of the back-illuminated image sensor.

Embodiment 2

Referring to FIG. 11 , this embodiment provides a method for reducing the dark current of a back-illuminated image sensor. Compared with the solution provided in the first embodiment, the difference of this embodiment is that in step 3), a top pinning layer 211 is further formed between the epitaxial layer 203 and the metal wiring layer 205 . The top pinned layer 211 can be formed by ion implantation on the epitaxial layer 203 from the front, and can also play a role in reducing dark current.

As shown in FIG. 11 , in addition to the above-mentioned epitaxial layer 203 , metal wiring layer 205 and top pinning layer 211 , the back-illuminated image sensor prepared in this embodiment also includes a pinning layer 202 and a layer formed on the epitaxial layer 211 . Photodiode 204 of layer 203 . A metal wiring layer 205 and a carrier 206 are also formed under the epitaxial layer 203 . A high-K dielectric layer 207 , a color filter layer 208 and a microlens structure 209 are also formed above the epitaxial layer 203 . A backside deep trench isolation structure 210 is also formed between the plurality of photodiodes 204 , and the high-K dielectric layer 207 is also filled therein.

As shown in FIG. 11 , this embodiment also provides a structure for reducing the dark current of a backside-illuminated image sensor. Compared with the structure shown in FIG. 10 in Embodiment 1, the difference between the structure provided in this embodiment is that in the A top pinning layer 211 is also formed between the epitaxial layer 203 and the metal wiring layer 205 .

Other implementations of this embodiment are the same as those of Embodiment 1, and are not repeated here.

Embodiment 3

Referring to FIG. 12 , this embodiment provides a method for reducing the dark current of a backside-illuminated image sensor. Compared with the solution provided in the first embodiment, the difference of this embodiment is that the high-K dielectric layer is not formed on the surface of the epitaxial layer 303 , and the dark current is reduced only by introducing the pinning layer 302 . This embodiment shows that only by introducing the pinning layer, the interface dark current can be reduced, the noise problem can be solved, and the imaging quality of the back-illuminated image sensor can be improved.

As shown in FIG. 12 , the backside illuminated image sensor prepared in this embodiment includes an epitaxial layer 303 , a metal wiring layer 305 , a pinning layer 302 and a photodiode 304 formed on the epitaxial layer 303 . A metal wiring layer 305 and a carrier 306 are also formed under the epitaxial layer 303 . A color filter layer 308 and a microlens structure 309 are also formed above the epitaxial layer 303 . A backside deep trench isolation structure 310 is also formed between the plurality of photodiodes 304 , which is filled with an insulating dielectric material, which may be a high-K dielectric material 307 .

As shown in FIG. 12 , this embodiment also provides a structure for reducing the dark current of a backside-illuminated image sensor. Compared with the structure shown in FIG. 10 in Embodiment 1, the difference between the structure provided in this embodiment is that in the No high-K dielectric layer is formed on the surface of the epitaxial layer 303 , and dark current is reduced only by introducing the pinning layer 302 .

Other implementations of this embodiment are the same as those of Embodiment 1, and are not repeated here.

In summary, the present invention provides a method and structure for reducing the dark current of a back-illuminated image sensor. The method includes the following steps: 1) providing an SOI substrate, where the SOI substrate includes a silicon substrate and a buried oxide layer. and top layer silicon; 2) forming a pinning layer formed by doping in the top layer silicon; 3) forming an epitaxial layer over the top layer silicon, and forming a photodiode in the epitaxial layer; 4) using the The buried oxide layer acts as a thinning stop layer for backside thinning of the silicon substrate. In the present invention, a pinned layer is formed on the top silicon of the SOI substrate, and the buried oxide layer is used as a thinning stop layer in the backside thinning process. By introducing the pinned layer as the charge trapping layer of the photodiode, the probability of the electric charge flowing into the photodiode to form dark current is reduced, and the pinned layer can be protected by the buried oxide layer in the silicon substrate thinning process. In addition, by cooperating with the backside growth of a high-K dielectric layer, the interface dark current is further reduced.

The above-mentioned embodiments merely illustrate the principles and effects of the present invention, but are not intended to limit the present invention. Anyone skilled in the art can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical idea disclosed in the present invention should still be covered by the claims of the present invention.

Claims (19)

1. A method of reducing dark current in a back-illuminated image sensor, comprising the steps of:

1) providing an SOI substrate, wherein the SOI substrate comprises a silicon substrate, a buried oxide layer and top silicon;

2) forming a pinning layer composed by doping in the top layer silicon;

3) forming an epitaxial layer above the top silicon layer and forming a photodiode in the epitaxial layer;

4) and taking the oxygen buried layer as a thinning stop layer, and thinning the back of the silicon substrate.

2. The method of reducing dark current in a back-illuminated image sensor of claim 1, wherein: the pinned layer has a doping type opposite to a doping type of the photodiode.

3. The method of reducing dark current in a back-illuminated image sensor of claim 1, wherein: the method of forming the pinning layer includes ion implanting the top layer silicon.

4. The method of claim 3, wherein the ion implantation has a dopant amount of 5 × 10¹²To 5 × 10¹⁴cm^-2。

5. The method for reducing dark current of a back-illuminated image sensor as claimed in claim 1, wherein the doping concentration of the epitaxial layer is 1 × 10¹⁴To 5 × 10¹⁵cm^-3The thickness of the epitaxial layer is 2.5 to 10 μm.

6. The method of reducing dark current in a back-illuminated image sensor of claim 1, wherein: the thickness of the oxygen burying layer is more than 1 mu m.

7. The method of reducing dark current in a back-illuminated image sensor of claim 1, wherein: after the photodiode is formed in step 3), a step of forming a metal wiring layer over the epitaxial layer is further included.

8. The method of reducing dark current in a back-illuminated image sensor of claim 7, wherein: a top pinning layer is also formed between the epitaxial layer and the metal wiring layer.

9. The method of reducing dark current in a back-illuminated image sensor of claim 1, wherein: before the back surface of the silicon substrate is thinned in the step 4), the method also comprises the step of bonding one surface of the SOI substrate, which is far away from the silicon substrate, on a slide glass.

10. The method of reducing dark current in a back-illuminated image sensor of claim 1, wherein: in step 4), the buried oxide layer is used as a thinning stop layer, and after the silicon substrate is thinned on the back side, the method further comprises the step of removing the residual buried oxide layer after the silicon substrate is thinned, and exposing the high-K dielectric layer, the color filter layer and the micro-lens structure which are sequentially formed on the pinning layer.

11. The method of reducing dark current in a back-illuminated image sensor of claim 10, wherein: the number of the photodiodes is multiple, and after the residual buried oxide layer after the thinning of the silicon substrate is removed, the method further comprises the step of forming a back deep trench isolation structure for separating the photodiodes in the epitaxial layer.

12. A structure for reducing dark current of a back-illuminated image sensor, comprising: the method comprises the following steps:

an epitaxial layer;

a photodiode formed in the epitaxial layer;

a top layer of silicon located over the epitaxial layer;

and the pinning layer is positioned in the top layer silicon and is formed by doping the top layer silicon.

13. The structure for reducing dark current of a back-illuminated image sensor of claim 12, wherein: the pinned layer has a doping type opposite to a doping type of the photodiode.

14. The structure for reducing dark current in a back-illuminated image sensor as claimed in claim 12, wherein the doping concentration of the epitaxial layer is 1 × 10¹⁴To 5 × 10¹⁵cm^-3The thickness of the epitaxial layer is 2.5 to 10 μm.

15. The structure for reducing dark current of a back-illuminated image sensor of claim 12, wherein: and a high-K dielectric layer, a color filter layer and a micro-lens structure are sequentially formed on the pinning layer.

16. The structure for reducing dark current of a back-illuminated image sensor of claim 15, wherein: and a metal wiring layer is also formed below the epitaxial layer.

17. The structure for reducing dark current of a back-illuminated image sensor of claim 16, wherein: a top pinning layer is also formed between the epitaxial layer and the metal wiring layer.

18. The structure for reducing dark current of a back-illuminated image sensor of claim 12, wherein: the number of the photodiodes is multiple, and a back deep trench isolation structure for separating the photodiodes is formed in the epitaxial layer.

19. The structure for reducing dark current of a back-illuminated image sensor of claim 12, wherein: the top layer silicon is a part of an SOI substrate, and the SOI substrate further comprises a silicon substrate and a buried oxide layer; the oxygen burying layer is positioned above the top silicon, and the silicon substrate is positioned above the oxygen burying layer; and when the back face of the silicon substrate is thinned, the buried oxide layer is used as a thinning stop layer.

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